This invention relates to AMTI synthetic aperture radars, and in particular to means for improving the clutter cancelling capabilities of the clutter cancelling circuits employed in such systems. Radar systems of the type to which the invention can be applied are disclosed in the patent of Sletten, Et Al, entitled "AMTI RADAR CLUTTER CANCELLING METHOD AND APPARATUS," U.S. Pat. No. 3,735,400, filed May 22, 1973; and in the copending patent application of William B. Goggins, Jr., entitled "METHOD AND APPARATUS FOR IMPROVING THE SLOWLY MOVING TARGET DETECTION CAPABILITY OF AN AMTI SYNTHETIC APERTURE RADAR," Ser. No. 563,009, filed 27 Mar. 1975. Radar systems of this type achieve clutter cancellation by receiving radar returns on multiple channels from receiving antennas disposed at discrete distances along the flight velocity vector of the AMTI radar bearing aircraft. The returns of some antennas are delayed in order to achieve return signal coincidence, and the processed returns of adjacent channels are subtracted to eliminate clutter. In order to achieve effective clutter cancellation, however, it is necessary in these and other state-of-the-art systems, that the physical antenna patterns of the system be carefully matched and that the synthetic aperture processors of each channel be exactly the same. Clutter cancellation in these systems can also be degraded or destroyed because of unknown, uncalibrated and time varying parameters.